HFU HF Underground
Technical Topics => Equipment => Topic started by: Telegrapher on March 14, 2019, 1912 UTC
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Hi fellow SWL and alike!
I have a quick question about a problem with connecting (any) kind of antenna to my radio. I used an RTL-SDR to give some technical view as images are the easiest way for me to show what I am struggling with at the moment.
It’s about the coaxial feed line plugs.
When I turn on my radio without antenna shows the first picture. Then the details of the cable and device. And the results of plugging the coax only into the antenna plug with its main center line (the core) and what happens when I attach the coax the way it must be. Results is no signal. Tested with several brand new cables without any results. So I am stuck at the moment and the only way to listen to shortwave is by only attaching the core of the coax cable without the screwing mechanism to hold the cable strong in its place. Resulting the cable to fall out of my radios antenna input every now and then. I need a fix for this. It’s strange. Please help me out. I would really appreciate it!
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https://imgur.com/gallery/C5vlmEz
Link in order with images
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The URL to your images seems to be bad, try again?
Based on your observations, it sounds like there could be a short between the center conductor and shield. How is the cable connected to your antenna? What type of antenna?
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Yep, sounds like a short, check the coax with a dmm.
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Hello, thanks for the quick response. I’ve just tested the plug of the feed line (which didn’t show any short circuit on the meter) secondly I tested the plug on my SDR (and there it is, a peak showed up on the meter so it has electrical current flowing from the center conductor to the shielding) the kind of plug I’m using is I think type n’ not sure what it’s called. It’s almost on all RTL-SDR dongles to be found. (Little nickel gold coated screw). I use a second plug to convert it to the bigger type used on a lot of CB-Radios (58rg coaxial cable) as it’s more stable and offers me the ability to hook the antenna up to multiple radios (as almost all my radios have the bigger type of plug on their back). So I am wondering if there’s a way to fix this. There’s nothing to see on the outside of the device. Always handled with care. The same problem is taking place with an older radio I have. If anyone has any tips or ideas to fix this issue I would love to hear it back. There’s nothing to see on the plugs of both radios in case of damage etc. so I think it’s maybe inside the cable itself that leads to the plug on the radio? Looks complicated to me screwing and disassembling the whole device for finding a solution that’s almost unseeable with my eyes. Will cleaning help with compressed air do the job maybe?
All tips and help is very appreciated! I’m just new into radio and I still have a lot to learn. Getting my hands on them is easy, but maintaining them is a whole new world yet to be explored as I have never build or repaired anything inside my radios. They always worked without trouble. A broken tube will be the first thing I might face in the future when I’ve finally set up the perfect antenna outdoors to listen all night long.
Kind regards
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Dmm each connector and adaptor, check each cable, you'll find the problem(s).
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Just tested a hour ago all connections. First the meter did show a few indications of short circuitry. But after I blew some air through each end of the lines it surprisingly went away. Maybe the screw wasn’t exactly on its location or maybe a little thin piece of leftover copper was hiding between the center and the braid. Well the first antenna line is fixed. The next one will be the line of my SDR.
I contacted a local guy I knew who has been working in the past with coaxial antennas and he said to me that if there was such a short circuit in the cable; then the radio wasn’t able to pick up any signal from it. Even with only the center conductor it wasn’t possible he said. This leads to a few more questions I want to get a clear answer for. I knew from the early months of my first antenna design that I was thinking that coax alone acts as an antenna (which is totally wrong as I’ve learned later on my journey) but I was able to pick up some stuff by coax only. Maybe the little end of the cable was just enough to pick radio China and Romania to lead me to think that coax was just all I needed back then. Funny noob story’s that lead to the more technical side as experimenting leads to solutions in funny ways. I love the hobby more and more every day :)
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On the use of coax as an antenna you run into a few things.
One is only the outer shield is impinged upon by the passing rf wave, so whatever length of coax you have, that's the total antenna aperture, not the inside center conductor PLUS the outer shield.
Another aspect is if you only connect the center conductor to the radio and no antenna elements on that same coax, the only way a signal can get to the center conductor from the outside is via capacitive coupling, and that will be very scant since only the very outer layer of the shield has rf flowing on it, not the inside facing surface of the shield.
It doesn't take much of a conductor to make for a decent vlf/mf/hf antenna, USN did a study and found that with a decent groundplane (ie a US warship plus the entire ocean) you only needed about 6 feet of wire, a coupling device to make the hi to lo z transformation would of course be spliced in between coax and antenna.
Case in point, I had a 1/4w cb antenna on top of some mast up about 10m, with a decent grounding wire run from base to ground stake. That thing was amazing from vlf to hf and on 6m too, of course I had to use a tuner for best results but it was an excellent dx antenna. Not very good at nvis at all but an amazing dxer.
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At the moment I use a dipole antenna called Gipsy made by windcamp. It’s about 3 meters up and length is 3.1 meters. Covering the 24MHz band as it’s strongest spot. I use this antenna for my ussr P-250M (currently listening to the Russian Buzzer (UVB-76) coming in very clear with little background noise. Due to limit of space I cannot stretch it out further. I’m still thinking about buying the PA0RDT mini whip later this year to see what that brings to the table. The only side effect is that I haven’t grounded the antenna outdoors. And the street lights give a bunch of noise when they turn on as it gets darker. Maybe there’s a way to limit the rf noise from surrounding objects. The first thing I’ll do is grounding the dipole this summer.
Anyway nice information about your experience and additional information about the CB antenna on 10m height. I think almost any kind of wire is suitable to use as an (temporary) antenna under the right circumstances. The possibilities are almost endless when it comes to designing.
Thanks for the nice response. It’s a pleasure to learn about new things to improve the art of building a project to the very finish. :)
Kind regards,
Telegrapher